A solid state power controller, SSPC, typically makes use of a digital microcontroller (or a DSP) to implement the required functionality with the aid of a software program. In this respect, it is the main object of this solid state power controller to switch electrical energy to a load and to interrupt the current flow in a time-dependent manner in the case of an overload.
Solid state power controllers of the prior art are in this respect programmed with the aid of their microcontrollers so that they have an I2t characteristic. This means that the product of the square of the current I and of the time t to reach a critical energy level is constant so that the time at which a disconnect occurs is inversely proportional to the square of the current. In other words, the apparatus is adapted to disconnect quickly at high currents, but to carry out a disconnect later at low currents. The advantage of this is that no disconnect is produced on a moderate overcurrent if the current is reduced or leaves the overcurrent range after a short time. This reduces the occurrence of disruptive disconnects, whereby incorrect tripping actions of disconnects are minimized. In addition, the solid state power controllers known in the prior art have a functionality that includes a memory function so that a preceding overload that has not affected a disconnect cooperates, after a transient departure from the overload range, with a second overload current that occurs briefly thereafter such that a disconnect of a line takes place faster than a disconnect that occurs only with respect to the presence of the second overload current (memory effect). This brings along the advantage that the thermal heating of the device to be protected by the solid state power controller can be mapped.
In summary, the solid state power controller therefore has the two main tasks for connecting or interrupting loads in an electric circuit as well as the protection of the cables connected to the load. The loads can in particular be electrical control actuators in aircraft, with a thermal circuit breaker still being very widespread in this environment. The protection in this respect comprises a short-circuiting and an overload observation with respect to an I2t curve of the cable.
It is necessary in aircraft to design the actuator power management unit (APMU) such that a protection of the cables connected to the APMU is provided. This protection should simultaneously also replace the behavior of a fuse which preferably has the typical I2t behavior and the previously used mechanical power interruption circuits. Advantages with respect to the flexibility and to a higher reliability are looked for here and the possibility of lowering costs is also seen.
As already stated further above, conventional solid state power controllers (SSPCs) use digital control units such as a microcontroller or a digital signal processor (DSP) to implement the required functions in the form of a software program. However, this architecture results in an increased error frequency and in an increase with respect to the certification effort for the implemented software since it has to be subjected to a particularly exact testing or certification in aircraft.
U.S. 2008/0174928 A1 discloses a solid state power controller that implements a I2t function by using a capacitor and a counter, with the first capacitor being charged several times when an overcurrent event occurs. The counter in this respect implements a count related to the number of charge cycles of the first capacitor to detect a shutdown condition. In addition, the circuit comprises a discharge module that is connected to the shutdown module and that comprises a resistor and a second capacitor, wherein an electrical parameter that is linked to the count drops over time when using the resistor and the second capacitor.
It is disadvantageous in the above-described model from the prior art that the capacitors used herein are of central importance and the overall accuracy and the long-term stability depends on these components. In addition, a thermal behavior is not taken into account precisely or reliably enough on the drop of the count.
The count is moreover only updated when an overload case is measured. The total energy balance that also looks at states in the non-overload case is not used.
It is therefore a goal of the present disclosure to provide a solid state power controller (SSPC) that overcomes the above-stated disadvantages and simultaneously achieves a I2t function without the presence of a microcontroller or the like.
The solid state power controller accordingly comprises a power switch for interrupting a line, a current sensor for measuring a current flow on the line, and a control unit for controlling the power switch and that is adapted to prevent an overcurrent on the line on the basis of the current measured by the current sensor. The control unit comprises a counter that is adapted to increment or decrement a count when the measured current is larger than a threshold value and to decrement or increment the count when the measured current is smaller than the threshold value, with the power switch being adapted to interrupt the line when the counter reaches or exceeds a predefined count limit value.
It is clear to the skilled person that the calculation operation (incrementing or decrementing) in the case of a current that is larger than the threshold value has to be different from the case in which the current is smaller than the threshold value. A pair of incrementing and decrementing accordingly always results.
The count of the counter may not be changed for the case in which the measured current IM is exactly equal to the threshold value ITH.
Unlike solid state power controllers known in the prior art, the value of the count directly depends on the measured current in the case of an overcurrent event. More reliable and more exact conclusions on the actual state of the line to be monitored thereby result since the charging of capacitors or the like cannot produce unwanted differences.
The power switch of the claimed solid state power controller substantially has the ability to disconnect or to connect the line to be monitored. In a separated state, the flow of a current through the line is suppressed, whereas in a connected state of the line a current flow through the line is possible. The current sensor measures the current strength of the current flowing on the line and forwards the measured current to the control unit. This is adapted to control the power switch, that is, the disconnecting or the closing of the line. This is done on the basis of the current measured by the current sensor in that a counter increments a count when the measured current is larger than a threshold value or decrements the count when the measured current is smaller than the threshold value. On a reaching or exceeding of a predefined count limit value, the power switch interrupts the line so that current can no longer flow through the line.
The control unit may comprise a comparator that is connected to the threshold value and to the measured current and that outputs a signal to an input of the counter that determines the count direction (incrementing or decrementing) of the count. An upward count direction corresponds to an incrementing, whereas a downward count direction corresponds to a decrementing. The comparator in this respect is a reliable variant to implement the required function of the control unit. It is possible by the wiring with the threshold value and with the measured current to generate a signal at the output of the comparator that adopts a defined state depending on whether the measured current is larger than the threshold value. The state is typically a high value or a low value of a voltage range. This signal is then forwarded to an input of the counter whose wiring decides on an incrementing or decrementing of the count. The output of the comparator for the case IM=ITH does not play any role, that is it can adopt a high value or a low value.
In accordance with a further optional further development of the present disclosure, the frequency of the counter at which it increments or decrements its count is related to a difference of the measured current from the threshold value, with this relationship being reflected by the expression (threshold value−measured current)2 to implement the typically required I2t characteristic of the solid state power controller.
The greater the difference of the measured current from the threshold value, the faster an incrementing or a decrementing of the counter takes place. If, for example, the measured current is considerably above the threshold value, an incrementing of the counter takes place quickly one after the other so that the count reaches or exceeds the predefined count limit value relatively quickly. If, in contrast, the measured current is just below the threshold value, this produces a decrementing of the count, with the time intervals between the individual decrementing steps being large.
In accordance with a further modification of the present disclosure, the control unit furthermore comprises a differentiator that is wired to the measured current and to the threshold value and that outputs a difference value between the threshold value and the measured current; a multiplier that multiplies the difference value output by the differentiator by itself (squares it); and a voltage frequency converter that converts the value output by the multiplier into a pulse sequence of a corresponding frequency, with the pulse sequence being forwarded to a clock input (CLK input) of the counter so that an incrementing or decrementing of the count takes place on each pulse.
The differentiator forms the difference between the measured current and the threshold value that is multiplied by itself in the multiplier. The sign of the difference is of no significance since the multiplier squares the difference value of the measured current and of the threshold value. The output of the multiplier is connected to the uF converter.
The voltage-to-frequency converter is a component that outputs a pulse sequence in dependence on its input voltage, the intervals of said pulse sequence being inversely proportional to the input voltage value. At a high input voltage value of the voltage-to-frequency converter, a pulse sequence having a high frequency is accordingly output, whereas at a low input voltage value of the voltage-to-frequency converter, a pulse sequence of low frequency is output. The pulse sequence output by the voltage-to-frequency converter is input at a clock input of the counter such that the counter carries out an incrementing or a decrementing at each of the pulses output by the multiplier.
Overall, this means that with a larger difference of the measured current from the threshold value, the count of the counter changes very fast, whereas with a small difference of the measured current and of the threshold value, the count of the counter only changes slowly over time, with the I2t characteristic being implemented by the squaring with the aid of the multiplier.
In accordance with a further embodiment of the present disclosure, the control unit furthermore comprises a current-to-voltage converter to convert the current measured by the current sensor into a voltage value. This current-to-voltage conversion is in this respect preferably carried out before a supply of a signal to a comparator and to a differentiator so that a voltage value is supplied to the differentiator and to the comparator that corresponds to the measured current.
In accordance with a further development of the present disclosure, the power switch for disconnecting a line is adapted to close the line on a falling below of the predefined count limit value or on a status below the predefined count limit value. A current can thereby flow through the line monitored by the solid state power controller provided that the count limit value of the counter has not been exceeded.
The counter may have a smallest count that can no longer be further decremented, with the smallest count being zero in one example.
In accordance with a further optional embodiment of the present disclosure, the control unit is adapted to map an I2t characteristic, with this mapping taking place without using a microcontroller in one example. This brings about advantages since it is not necessary to subject the software of a microcontroller or of a DSP to a certification test in critical applications; for example on a use of a solid state power controller in an aircraft.
It is additionally possible that the solid state power controller does not comprise a microcontroller and/or substantially only comprises the components of the paragraphs above describing the solid state power controller.
In accordance with a variation of the present disclosure, the power switch is bidirectionally conductive and may comprise an IGBT, a MOSFET, a Si semiconductor and/or a SiC semiconductor.
The present disclosure furthermore comprises a solid state power controller arrangement that comprises at least two of the above-described solid state power controllers, with a first solid state power controller being present in a line that serves the conducting of a current to a load, and with a second solid state power controller being present in a line that serves the leading away from the load. This two-channel architecture allows the detection of a ground fault or of other defects that are based on a differential current imbalance without any greater effort. One solid state power controller is therefore inserted into the positive line and one solid state power controller into the negative line of a load to be monitored and/or to be connected with the solid state power controller.
Further advantages, details and features of the present disclosure will become clear with reference to the following discussion of the Figures.